In 3rd generation WCDMA systems, the downlink capacity is a limiting factor for system capacity. Therefore, site selection diversity transmit power control (SSDT) has been proposed as a macro-diversity method for the soft handover mode in radio access networks.
During soft handover, a terminal device, such as a mobile station or user equipment, is in the overlapping cell coverage area of e.g. two sectors belonging two different base stations, which are called Node Bs in the corresponding 3rd generation partnership project (3GPP) specifications. The terminal device monitors received signals broadcasted from the different base stations, compares them to a set of thresholds, and reports them accordingly back to the base stations. Based on this information, the network orders the terminal device to add or remove base station links from its active set of soft handover cells. The active set is defined as a set of base stations or active cells from which the same user information is sent to the user equipment (UE). Furthermore, in a micro diversity or softer handover case, soft handover is performed between sectors or cells belonging to the same base station or node B. Thus, in the present example, the communications between the mobile station and the base station may take place concurrently via two air interface channels, one for each sector or active cell separately. This requires the use of two separate codes in the downlink direction, so that the mobile station can distinguish the signals. The SSDT operation can be summarized as follows. The mobile station selects at least one of the cells from its active set to be “primary”, all other cells are classed as “secondary”. The main objective is to transmit on the downlink from the primary cell, thus reducing the interference caused by multiple transmissions in the soft handover mode. A second objective is to achieve fast site selection without network intervention on higher protocol layers, thus maintaining the advantage of the soft handover.
In order to select at least one primary cell, each cell is assigned a temporary identification (ID) and the mobile station periodically informs a primary cell. ID to the active cells. In response thereto, the non-primary cells selected by the mobile station switch off their transmission power. The primary cell ID is delivered by the mobile station to the active cells via an uplink FBI (Feedback Information) field. Thus, each cell is given a temporary ID during SSDT and this ID is utilized as a site selection signal. The ID is given a binary bit sequence and the ID codes are transmitted aligned to the radio frame structure.
The mobile station selects a primary cell periodically by measuring the Received Signal Code Power (RSCP) of common pilot channels (CPICHs) transmitted by the active cells. The cell with the highest CPICH RSCP is selected as a primary cell. Also the Signal-to-Interference Ratio (SIR) could be used for primary cell selection. The mobile station periodically sends the ID code of the primary cell via predetermined portions of the uplink FBI field assigned for SSDT use (FBI S field). A cell recognizes its state as non-primary if the following conditions are fulfilled simultaneously:    1. the received ID code does not match with the own ID code;    2. the received uplink signal quality satisfies a quality threshold Qth defined by the network; and    3. if uplink compressed mode is used, less than NID/3 bits are lost from the ID code (as a result of uplink compressed mode), where NID denotes the number of bits in the ID code.
Otherwise, the cell recognizes its state as primary.
The state of the cells (primary or non-primary) in the active set is updated synchronously.
Thus, in SSDT, a mobile station periodically chooses at least one of its active cells or base stations having minimum path loss in its transmission to the mobile station. However, since the ID is sent over the air interface, it may be possible that the ID is detected erroneously. When the ID is detected erroneously, the problem may occur that all active base stations switch off their output power simultaneously. On the other hand, the mobile station may receive a downlink transmission signal from an assumed primary base station, which however has not transmitted the data, while it was transmitted by a secondary base station. The first problem may cause frame errors but does not lead to additional interference, whereas the latter problem is a more serious problem, because in this situation, the fast transmit power control takes up the control of the transmission power of the undesired secondary base station. The resulting high transmission power of the undesired secondary base station might cause high additional interference to other users.